Single phase AC/DC converter with power factor correction has an issue of pulsating input power at double line frequency. This pulsating power usually is stored in an intermediate capacitor if output power is DC. The required capacitance, depending on the ripple power at double line frequency, will result in either a short lifetime with electrolytic capacitors or high volume with film capacitors. Especially when wide bandgap semiconductors have been used to shrink other passive components significantly, the DC link capacitor becomes one major power density barrier.
In some applications, such as automobile and aviation electronics, both long lifetime and high power density are required, therefore some efforts have been made to address this issue. Given the same ripple energy, capacitance is reduced by enlarging the capacitor voltage ripple. This concept can be implemented directly to the DC link capacitor, as shown in a grid-interface bi-directional converter, which increases device voltage stress and the capacitance reduction is limited due to the range limitation of DC link voltage swing. Alternatively, it can be implemented in the auxiliary capacitors, but the realization is more complicated and expensive.
Recent research from Lithium-Ion batteries shows that a charging current with two times the line frequency ripple causes no harm to the battery at least in the short term. Therefore, proposed charger designs with the charging current containing low frequency ripples, so the DC link capacitance can be significantly reduced.
However, with sinusoidal charging, the switches usually suffer from hard-switching, at least at the valley of the charging current for most charger topologies, unless other components are added to produce a smooth current/voltage transitions at the switching moment. Therefore, the overall efficiency under this condition is impacted. An example of the charger topology is shown in FIG. 1, which is comprised of a full-bridge AC/DC stage plus a dual active bridge DC/DC stage. The DAB topology is advantageous in terms of fixed frequency, soft-switching and symmetrical configuration for hi-directional power flow. Advanced DAB modulation for an extended zero-voltage switching range has been intensively investigated, but the scheme is very complicated.